Technical Field
The embodiments herein generally relate to optics, and more particularly to optical collimators.
Description of the Related Art
High Energy Laser (HEL) transmitters using electrical power may have a remarkable niche in a military soldier's arsenal owing to: a) extremely precise target selection without collateral damage; b) delivery of damaging energy at the speed of light; c) easy scalability of damaging factor using a relatively small laser energy focused into very small spot; d) the virtually unlimited number of very cheap shots. There is an applicability of HEL directed energy systems in shooting at light targets., such as, UAVs, mortars, small boats, and missiles. However, the demonstrated apparatus is bulky and heavy, and requires a heavy-weight carrier, such as a truck or ship.
It is a challenge to miniaturize the entire HEL system for military tactical use (over short distances of 1-3 km) to a level of Size Weight and Power (SWaP) sufficiently small to be carried by small trucks like a Humvee, or even as a man-carried device.
The following issues should be considered for miniaturization of an HEL system: 1) Maximum efficiency of conversion of electrical power to optical radiation. 2) Heat management: a) during the conversion of wall outlet (battery) electric power into laser radiation, and b) parasitic radiation management within the transmitter. 3) High quality of the laser beam to allow focusing energy onto the target with the smallest (diffraction limited) spot size. 4) Maximizing the fraction of laser power directed toward the target (minimizing the parasitic losses). 5) Mitigation of beam wander and degradation caused by propagation through turbulent atmospheres and platform jitter. The above-mentioned issues 2b), 3), 4) and 5) are directly related to the transmitter utilizing the high-quality laser radiation (monochromaticity, coherence, power, etc.) created by modem lasers or laser amplifiers.
Solid state lasers (SSL) have very high wall-plug efficiency exceeding 60%, but SSL tend to have poor beam quality. After improvement of beam quality for effective focusing on the target the ultimate wall-plug SSL efficiency drops to less than 25%. The rest of the power (>75%) is mostly transformed to parasitic heat inside the laser source, and extensive resources are necessary to dissipate this heat (heavy chillers, cooling fluids through pumping diodes, etc.). The increase of output optical apertures for improvement of beam focusing is accompanied with an increase of anisoplanatic contribution of high-aperture optics and requires extra apparatus; e.g., adaptive optics with inevitable size, weight and power (SWaP) increase.
The fiber lasers are considered as the most advanced laser sources owing to very high wall-plug efficiency reaching 40% and almost ideal beam quality, M2<1.1. However, the increase of power of fiber laser beam with conservation of high quality is restricted with non-linear effects in single-mode fibers; e.g., SBS (Stimulated Brillouin Scattering). The use of multi-mode fibers for increase of power radiation, above 10 kW leads to loss of beam quality and to the necessity of increasing the size of focusing mirrors, addition of adaptive optics with increase of SWaP, and with slowing down the speed of targeting.
The practical way to scale the power to 100 kW or above is combining multiple fiber lasers each with modest power and high beam quality. The weaponry level can be achieved by combining tens of fiber lasers with power 1-2 kW each. Different methods of laser combination have been explored by the conventional solutions. The conformal array of fiber laser collimators (sub-apertures) is among the most suitable for mobile applications.
The size of the diffraction limited spot, Wf, focused on a target is an important criterion of collimator performance; hence, the lens should have a large diameter to provide a smaller spot. For high-power fiber laser radiation emitted by a single mode fiber, the divergence angle of the beam is usually very small and the “beam angle” containing the “Gaussian power” (86.5% of full power) can be as small as 4°. As used herein, the divergent beam is referred to as a “Gaussian beam”, owing to that in most cases the approximation of intensity distribution of the beam emitted, from fiber tip can be close enough to a Gaussian distribution. The “Gaussian” approximation allows one to calculate simply the parameters of the optical system based on fiber optics with sufficiently good accuracy for the following practical fabrication of such a system. With the increase of lens diameter d the focal length F should be also increased, or m additional negative lens should be placed between the fiber tip and output lens. Both approaches require either the increase of length or complexity of the collimator.
Generally, for high performance of high-power fiber laser collimators and especially for arrays of such collimators the “Gaussian fill factor” (i.e., the ratio of the diameter of the output lens to the Gaussian beam diameter) and “lenslet fill factor” (i.e., the ratio of the diameter of the output lens to the distance between adjacent fiber tips) have relatively small range for acceptable variations. The best performance of an array requires a high density of fiber collimators (sub-apertures) with ideally 100% lens fill factor, and significant truncation of the Gaussian beams emitted by fiber tips. The accepted truncation should be no less than 5% even with a 100% fill factor of the output lens assembly in case of coherent beam combining (CBC). Tins truncated radiation may carry very high power IkW or higher even for array of seven sub-apertures, each carrying for instance 1.5 kW or more. Standard “interception-dissipation-cooling” approaches to this parasitic radiation requires the extensive cooling of array inner structure, which is not consistent with development of compact and mobile laser transmitters.